Research on aspartate transcarbamylase (ATCase) is aimed at determining the relationship between tertiary and quaternary structures and the effect of subunit interactions on the folding of polypeptide chains. Hence we are studying the quaternary structure of this regulatory enzyme composed of twelve polypeptide chains by examining both the subunit interactions and the strengths of bonding domains between the different subunits. For this work the assembly of the oligomeric enzyme comprising two catalytic trimers and three regulatory dimers is being studied through the use of radioactively labeled subunits and kinetic measurements. Through these experiments various incomplete enzyme-like molecules have been identified as intermediates in the total assembly process. The rate of rupture of the bonding domains is being studied through exchange experiments and disproportionation studies using a less stable form of the enzyme lacking one regulatory subunit. For most of this work the native enzyme and its subunits are being employed but in addition studies are also being conducted on hybrids composed of native and chemically modified enzyme molecules. Moreover various mutant enzymes have now been purified so that the same types of experiments can be conducted on proteins that are not chemically modified. The homotropic and heterotropic effects characteristic of allosteric enzymes were shown to be manifested by molecules having the general architecture of the native enzyme. Even though all of the catalytic chains in one subunit were inactive, the molecule that was constructed still showed the cooperativity characteristic of ATCase. In addition it was shown with other hybrids that not all of the chains in either subunit must be active to endow the complex with allosteric properties. Thus far the homotropic and heterotropic effects have been shown to be inextricably interlinked. In the course of this work it was found that a hybrid containing one wild-type catalytic subunit and one mutant, inactive subunit, along with the three native regulatory subunits, was relatively inactive. This "paralysis" is now the subject of detailed study in terms of the strengths of the bonds between catalytic and regulatory chains in the various mutants. Another aspect of the work concerns the assembly of functional catalytic trimers from independent, unfolded polypeptide chains.